Along with soulful eyes, endearingly long necks and warm fuzzy coats, llamas have a far less appreciated feature: They make an array of immune system antibodies so tiny they can fit into crevices on the surface of an invading virus.

That feat could one day protect humans from families of flu viruses that bedevil scientists with their unpredictable and shape-shifting ways.

All, potentially, with a once-a-year puff up the nose.

In a study in Friday’s edition of the journal Science, a team from the Scripps Research Institute in La Jolla and their international colleagues have taken a major step toward the long-sought goal of developing a universal vaccine against influenza.

When they tested their intranasal formulation in mice, it quickly conferred complete protection against a raft of human flu strains adapted to mice. Those include A viruses, such as the H1N1 “swine flu” that touched off a global pandemic in 2009, and B viruses, which occur only in humans.

Against H1N1, a dose of the experimental vaccine was shown to protect for at least 35 days — a span of time equivalent to more than a flu season for humans.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, offered a full-throated appreciation for the study, which received funding from the National Institutes of Health.

“From a scientific and technical standpoint, this is really a very elegant study — the highest quality of science,” Fauci said. He praised it for demonstrating that in order to protect people from pathogens that can change or emerge unpredictably, scientists must construct vaccines that can knock down an array of viruses, even in people whose immune systems are fragile or compromised.

Influenza is a viral scourge that kills as many as 650,000 people each year, according to the World Health Organization. To fight it, the research team borrowed techniques from immunology, microbiology, nanotechnology and genetic engineering labs around the world.

First, they vaccinated llamas against a number of A and B strains of influenza. Then they took blood samples to collect the antibodies the llamas produced in response.

Among them were four uniquely small antibodies that showed an ability to destroy many strains of influenza. In a nod to their size and function, they called their creations “nanobodies.”

From those multitasking little powerhouses, the researchers engineered a protein capable of squeezing into spaces on a virus’ surface that are too small for most proteins. The resulting “multidomain antibody MD3606,” with its “impressive breadth and potency,” could confer protection against pretty much any strain of flu that nature could throw in humankind’s way, the study authors said.

If the dominant strain in a given season were to suddenly change, these antibodies would be ready for the unwelcome guest. If a flu strain came out of nowhere and threatened a population with no immunity to it — the nightmare scenario of pandemic flu — this supercharged defender would recognize that flu and counter it. If health officials guessed wrong about what flu strain was coming and ordered a vaccine that would be largely ineffective — a scenario that played out last flu season — this package of antibodies could save the day.

But the researchers faced a key hurdle: getting the human immune system to make such a super-protein even when it’s weighed down by age, stress and disease.

Their solution: Don’t try.

Instead, they devised a way to work around humans’ unreliable response to vaccines, building a gene that encoded the production plans for their powerhouse protein. To ferry that gene into a host organism, they enlisted a harmless virus used by labs working on gene therapy.

By splicing their designer gene into this viral delivery device, the scientists not only found a way to get their antibody package into a host, they were delivering the manufacturing machinery to produce it. This “passive transfer” of antibodies gives this vaccine candidate the potential to be equally effective in everyone, Fauci said.

The next step is to conduct further tests in animals and clinical trials in humans, and that “will take years,” he said. “But if fully successful — a majestic leap right now — it could essentially eliminate the need from season to season” to divine which of countless possible flu viruses will rear up, and to then build a yearly flu vaccine that neatly fits the bill.

Scripps immunologist Ian Wilson, the study’s senior author, said that as the cells “infected” by the delivery virus turn over, repeated doses might be needed to sustain the production of antibodies. “We don’t really know how long this treatment would survive in humans yet,” he said.

But even less-than-permanent immunity against a broad range of flu threats would help buffer people from the emergence of unexpected flu strains, Wilson said. And the rapid response of mice to the vaccine suggests it could be used to inoculate a population after a new viral threat has emerged, he added.

That the experimental vaccine might need to be administered each year makes it an interesting hybrid, said Ted M. Ross, who directs the University of Georgia’s Center for Vaccines and Immunology.

“This approach is similar to antivenom,” Ross said. “The therapeutic is an antibody that was made in another species to neutralize the toxin. It’s short-term, but it gets you through the period of time where bad things could happen.”

Over time, patients who got the same antibodies repeatedly might start to build resistance to them, he said. Vaccine makers could counter that by finding and including new antibodies in their formulation every few years, he suggested.